Manganese dioxide is known as a cathode active material for e.g. manganese dry cells or alkali-manganese dry cells and has advantages such that it is excellent in storage stability and inexpensive. In particular, the alkali-manganese dry cells employing manganese dioxide as a cathode active material, are excellent in the discharge characteristics under heavy loading. Thus, they are widely used in applications ranging from digital cameras, portable tape recorders and portable information apparatus to game machines and toys, and there is a rapidly growing demand for them.
However, alkali-manganese dry cells have had a problem such that the utilization rate of manganese dioxide as a cathode active material decreases with the increase of the discharge current and manganese dioxide cannot use in such low discharge voltage condition, whereby the discharge capacity will be substantially impaired. That is, the alkali-manganese dry cells have had a drawback that when they are used in apparatus using a large current (for high rate discharge), manganese dioxide packed as a cathode active material is not sufficiently utilized, and the useful time is short.
Therefore, excellent manganese dioxide capable of providing a high capacity and long useful life even under such a high rate pulse discharge condition that a large current is taken out in a short period of time, i.e. manganese dioxide excellent in a so-called high rate discharge characteristic, has been desired.
Heretofore, in order to improve the high rate discharge characteristic, production of electrolytic manganese dioxide having a high potential (hereinafter referred to as alkali potential) as measured in a 40 wt % KOH aqueous solution by using a mercury/mercury oxide reference electrode as a standard, has been proposed (Patent Documents 1 to 3 and Non-Patent Document 1).
Further, along with energy saving of digital apparatus in recent years, the characteristics under discharge current (for middle rate discharge) lower than the high rate discharge are required, and electrolytic manganese dioxide excellent not only in the high rate discharge characteristic but also in the middle rate discharge characteristic has been desired.
As a method for producing electrolytic manganese dioxide having improved high rate discharge characteristic, a method for producing electrolytic manganese dioxide by heating electrolytic manganese dioxide before neutralization to reduce the amount of use of sodium has been proposed (Patent Document 4). However, as the crystal structure of the electrolytic manganese dioxide is changed by heating, the high rate discharge characteristic of the electrolytic manganese dioxide to be obtained is insufficient. Further, by this production method, neutralization of the electrolytic manganese dioxide is insufficient since the amount of use of sodium is reduced, and such may cause corrosion of metal materials in the interior of the dry cell.
Further, the high rate discharge characteristic is improved by controlling the sulfuric acid amount, and a method for producing electrolytic manganese dioxide by controlling the surface sulfuric acid amount to be at least 0.10 wt % (Patent Document 5) and a method for producing manganese dioxide by adjusting the sulfate group content to be from 1.3 wt % to 1.6 wt % (Patent Document 6) have been proposed. However, since the electrolytic manganese dioxide obtained by such a production method contains a large amount of sulfuric acid, not only deterioration of the dry cell during storage results or the cell voltage tends to be unstable, but also corrosion of metal materials in the production apparatus or the interior of the dry cell may occur.
On the other hand, in order to improve the high rate discharge characteristic without a problem of the corrosion of the metal materials, a production method of controlling the MS-pH and the sulfate group content of the electrolytic manganese dioxide and further controlling the particle size of the electrolytic manganese dioxide and the sodium content has been proposed (Patent Document 7). However, although the electrolytic manganese dioxide obtainable by this production method is less likely to cause corrosion of the metal materials and is excellent in the high rate discharge characteristic, its middle rate discharge characteristic is still insufficient.
Further, electrolytic manganese dioxide excellent in the high rate discharge characteristic obtained by an electrolysis method by changing the sulfuric acid concentration during the electrolysis period (Patent Document 3) and electrolytic manganese dioxide excellent in the high rate discharge characteristic obtained by treating the electrolytic manganese dioxide after electrolysis with sulfuric acid (Patent Document 8) have been proposed, however, their middle rate discharge characteristic is still insufficient.
As described above, heretofore, electrolytic manganese dioxide excellent in both high rate discharge characteristic and middle rate discharge characteristic, which does not cause corrosion of the metal materials, has not been obtained yet.